The present invention relates to a method of metal detection by means of a coil device having several, separately controllable regions, as well as to a measuring device for the metal detection.
For the purpose of monitoring safety in airports, gate frames are used which are provided with coil devices in the lateral parts of the frame for metal detection. When people pass through the gate frames, it is therefore possible to check whether the people are carrying metal objects with them. If a metal object is detected, the measuring device which is connected with the coil device of the gate frame releases an acoustic and/or visual signal.
In order not only to determine the presence of a metal object but also to provide a signal where the fixed metal object is located in the space enclosed by the gate frame, the coil device is provided with different, separately controllable regions. The transmitting coil and the receiving coil having different regions can be controlled or evaluated separately so that it can be determined in which spatial region a detected metal object is located. The control of the different transmitter coils in such a coil arrangement may be offset in time in order to enable a spatial association with respect to the received measuring signal. The timely offset control of the different transmitting coils has however the disadvantage that always only that spatial region inside the gate frame is monitored whose associated transmitting coil is directly controlled. If a person moves relatively fast through the gate frame a metal object which is carried with him can be not detected since it can move through a spatial region for the associated transmitting coil is not directly controlled. If for example in a certain space five metal detectors are used each provided with 20 coils, the pulse repeat rate of each coil is reduced by the factor 5.times.20=100 for an individual coil arrangement. The power/noise ratio is therefore worsened by the factor .sqroot.100=10.
A known measuring process with electronic object analysis is disclosed in the article "Ein neuartiges Metallsuchgerat nach dem Pulsverfahren fur grosse Gelande-flachen mit elektronischer Objektanalyse und -auswahl" in Fachzeitschrift Frequenz, 22 (1968) 10, pages 300-305.